Reactions of 5-methylcytosine cation radicals in DNA and model systems: thermal deprotonation from the 5-methyl group vs. excited state deprotonation from sugar.

Abstract:

PURPOSE:To study the formation and subsequent reactions of the 5-methyl-2'-deoxycytidine cation radical (5-Me-2'-dC•(+)) in nucleosides and DNA-oligomers and compare to one-electron oxidized thymidine. MATERIALS AND METHODS:Employing electron spin resonance (ESR), cation radical formation and its reactions were investigated in 5-Me-2'-dC, thymidine (Thd) and their derivatives, in fully double-stranded (ds) d[GC*GC*GC*GC*](2) and in the 5-Me-C/A mismatched, d[GGAC*AAGC:CCTAATCG], where C* = 5-Me-C. RESULTS:We report 5-Me-2'-dC•(+) production by one-electron oxidation of 5-Me-2'-dC by Cl(2)•- via annealing in the dark at 155 K. Progressive annealing of 5-Me-2'-dC•(+) at 155 K produces the allylic radical (C-CH(2)•). However, photoexcitation of 5-Me-2'-dC•(+) by 405 nm laser or by photoflood lamp leads to only C3'• formation. Photoexcitation of N3-deprotonated thyminyl radical in Thd and its 5'-nucleotides leads to C3'• formation but not in 3'-TMP which resulted in the allylic radical (U-CH(2)•) and C5'• production. For excited 5-Me-2',3'-ddC•(+), absence of the 3'-OH group does not prevent C3'• formation. For d[GC*GC*GC*GC*](2) and d[GGAC*AAGC:CCTAATCG], intra-base paired proton transferred form of G cation radical (G(N1-H)•: C(+ H(+))) is found with no observable 5-Me-2'-dC•(+) formation. Photoexcitation of (G(N1-H)•:C(+ H(+))) in d[GC*GC*GC*GC*](2) produced only C1'• and not the expected photoproducts from 5-Me-2'-dC•(+). However, photoexcitation of (G(N1-H)•:C(+ H(+))) in d[GGAC*AAGC:CCTAATCG] led to C5'• and C1'• formation. CONCLUSIONS:C-CH(2)• formation from 5-Me-2'-dC•(+) occurs via ground state deprotonation from C5-methyl group on the base. In the excited 5-Me-2'-dC•(+) and 5-Me-2',3'-ddC•(+), spin and charge localization at C3' followed by deprotonation leads to C3'• formation. Thus, deprotonation from C3' in the excited cation radical is kinetically controlled and sugar C-H bond energies are not the only controlling factors in these deprotonations.

journal_name

Int J Radiat Biol

authors

Adhikary A,Kumar A,Palmer BJ,Todd AD,Heizer AN,Sevilla MD

doi

10.3109/09553002.2014.884293

subject

Has Abstract

pub_date

2014-06-01 00:00:00

pages

433-45

issue

6

eissn

0955-3002

issn

1362-3095

journal_volume

90

pub_type

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